Separator, preparation method therefor and related secondary battery, battery module, battery pack and device

ABSTRACT

A separator, comprising a substrate and a coating provided on at least one surface of the substrate are provided. In some embodiments, the coating comprises inorganic particles and organic particles, the organic particles comprise first organic particles and second organic particles both embedded in the inorganic particles and forming protrusions on the surface of the coating; the first organic particles are secondary particles, and the number of the first organic particles in the coating is denoted as A; the second organic particles are primary particles and the number of the second organic particles in the coating is denoted as B; with the separator satisfying: 0.05≤A/B&lt;1. The present application also relates to a method for preparing the separator, as well as a secondary battery comprising the separator, a battery module, a battery pack and a device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International Application PCT/CN2020/132956, filed Nov. 30, 2020 and entitled “SEPARATOR, PREPARATION METHOD THEREFOR AND RELATED SECONDARY BATTERY, BATTERY MODULE, BATTERY PACK AND DEVICE”, which is incorporated herein. by reference in its entirety.

TECHNICAL FIELD

The present application belongs to the technical field of secondary batteries, and in particular relates to a separator, a preparation method therefor and a related secondary battery, battery module, battery pack and device.

BACKGROUND ART

Secondary batteries are widely used in various consumer electronic products and electric vehicles due to their outstanding features, such as a light weight, no pollution and no memory effect.

With the continuous development in the new energy industry, higher requirements for the use of secondary batteries have been proposed by costumers. For example, the energy density of secondary batteries is designed to be higher and higher; however, the increase of the energy density of the battery is often detrimental to balancing dynamic performance, electrochemical performance, or safety performance.

Therefore, it is a key challenge in the field of battery design to create batteries with a good rate performance, storage performance and safety performance.

SUMMARY OF THE INVENTION

In view of the technical problems mentioned in the background art, in a first aspect, the present application provides a separator, which aims to enable a secondary battery comprising the separator to have good rate performance, storage performance and safety, performance.

In order to achieve the above object, in a first aspect, the present application provides a separator, comprising: a substrate and a coating provided on at least one surface of the substrate; wherein the coating comprises inorganic particles and organic particles; the organic particles comprise first organic particles and second organic particles both embedded in the inorganic particles and forming protrusions on the surface of the coating; the first organic particles are secondary particles, and the number of the first organic particles in the coating is denoted as A; the second organic particles are primary particles, and the number of the second organic particles in the coating is denoted as B; with the separator satisfying: 0.05≤A/B<1.

Compared with the prior art, the present application comprises at least the following beneficial effects:

the separator of the present application comprises inorganic particles and organic particles in the same coating, which greatly reduces the thickness of the separator, thereby improving the energy density of the battery; moreover, the organic particles comprise the first and the second organic particles in a specific distribution state and in a number ratio range, which can not only effectively improve the bonding between the separator and the electrode plate, but also reduce the wrinkling of the separator during the cycling of the battery, such that the battery can have safety performance and rate performance.

In any embodiments of the present application, the separator satisfies: 0.2≤A/B≤0.6; optionally, 0.25≤A/B≤0.5. When A/B is within the given range, the rate performance, storage performance and safety performance of the battery can be further improved.

In any embodiment of the present application, the first organic particles have a number-average particle size of ≥8 μm; optionally, the first organic particles have a number-average particle size of 12 μm-25 μm. When the number-average particle size of the first organic particles is within the given range, the rate performance of the battery can be further improved.

In any embodiments of the present application, the second organic particles have a number-average particle size of ≥2 μm; optionally, the second organic particles have a number-average particle size of 3 μm-7 μm. When the number-average particle size of the second organic particles is within the given range, the rate performance and safety performance of the battery can be further improved.

In any embodiment of the present application, the ratio of the number-average particle size of the first organic particles to that of the second organic particles is ≥2.0 μm; optionally, the ratio of the number-average particle size of the first organic particles to that of the second organic particles is 2.5-4.0. When the ratio of the number-average particle size of the first organic particles to that of the second organic particles is within the given range, the rate performance and safety performance of the battery can be further improved.

In any embodiments of the present application, the mass percentage of the inorganic particles in the coating is ≤75%, optionally, the mass percentage of the inorganic particles in the coating is 60%-75%.

In any embodiments of the present application, the mass percentage of the first organic particles in the coating is ≥12%, optionally, the mass percentage of the first organic particles in the coating is 15%-25%.

In any embodiments of the present application, the mass percentage of the second organic particles in the coating is 10%, optionally, the mass percentage of the second organic particles in the coating is 2%-10%.

When the organic particles, the first organic particles or the second organic particles is within the above range, these three types of particles can have a better synergistic effect, thereby further improving the energy density and the cycling performance of the battery.

In any embodiment of the present application, the separator has a single-sided coating weight per unit area of ≤3.0 g/m²; optionally, the separator has a single-sided coating weight per unit area of 1.5 g/m²-3.0 g/m². When the weight of single-sided coating on the separator per unit area is within the given range, the energy density of the battery can be further improved while ensuring the cycling performance and safety performance of the battery.

In any embodiment of the present application, the first organic particles comprise one or more of a homopolymer or copolymer of a fluorine-containing olefine monomeric unit, a homopolymer or copolymer of an olefine monomeric unit, a homopolymer or copolymer of an unsaturated nitrile monomeric unit, a homopolymer or copolymer of an alkylene oxide monomeric unit, and modified compounds of these homopolymers or copolymers.

In any embodiment of the present application, the first organic particles comprise one or more of polytetratluoroethylene, polychlorotrifluoroethylene, polyvinyl fluoride, polyvinyli dene fluoride, polyethylene, polypropylene, polyacrylonitrile, polyethylene oxide, a copolymer of different fluorine-containing olefine monomeric units, a copolymer of a fluorine-containing olefine monomeric unit and an olefine monomeric unit, a copolymer of a fluorine-containing olefine monomeric unit and an acrylic monomeric unit, a copolymer of a fluorine-containing olefine monomeric unit and an acrylate monomeric unit, and modified compounds of these homopolymers or copolymers.

In any embodiment of the present application, the first organic particles comprise one or more of a vinylidene fluoride-trifluoroethylene copolymer, a vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene fluoride-trifluoroethylene-hexafluoropropylene copolymer, a vinylidene fluoride-hexafluoropropylene-acrylic acid copolymer, a vinylidene fluoride-hexafluoropropylene-acrylate copolymer, and modified compounds of these copolymers.

In any embodiment of the present application, the second organic particles comprise one or more of a homopolymer or copolymer of an acrylate monomeric unit, a homopolymer or copolymer of an acrylic monomeric unit, a homopolymer or copolymer of a styrene monomeric unit, a polyurethane compound, a rubber compound, and modified compounds of these homopolymers or copolymers.

In any embodiment of the present application, the second organic particles comprise one or more of a copolymer of an acrylate monomeric unit and a styrene monomeric unit, a copolymer of an acrylic monomeric unit and a styrene monomeric unit, a copolymer of an acrylic monomeric unit, an acrylate monomeric unit and a styrene monomeric unit, a copolymer of a styrene monomeric unit and an unsaturated nitrile monomeric unit, a copolymer of a styrene monomeric unit, an olefine monomeric unit and. an unsaturated nitrile monomeric unit, and modified compounds of these copolymer.

In any embodiment of the present application, the second organic particles comprise one or more of a butyl acrylate-styrene copolymer, a butyl methacrylate-isooctyl methacrylate copolymer, an isooctyl methacrylate-styrene copolymer, a methacrylate-methacrylic acid-styrene copolymer, a methyl acrylate-isooctyl methacrylate-styrene copolymer, a butyl acrylate-isooctyl acrylate-styrene copolymer, a butyl acrylate-isooctyl methacrylate-styrene copolymer, a butyl methacrylate-isooctyl acrylate-styrene copolymer, a butyl methacrylate-isooctyl methacrylate-styrene copolymer, a styrene-acrylonitrile copolymer, a styrene-butadiene-acrylonitrile copolymer, a methyl acrylate-styrene-acrylonitrile copolymer, an isooctyl methacrylate-styrene-acrylonitrile copolymer, a styrene-vinyl acetate copolymer, a styrene-vinyl acetate-pyrrolidone copolymer, and modified compounds of these copolymers.

In any embodiments of the present application, the inorganic particles comprise one or more of boehmite (γ-AlOOH), aluminum oxide (Al₂O₃), barium sulfate (BaSO₄), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)₂), silicon dioxide (SiO₂), tin dioxide (SnO₂), titanium oxide (TiO₂), calcium oxide (CaO), zinc oxide (ZnO), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), nickel oxide (NiO), cerium oxide (CeO₂), zirconium titanate (SrTiO₃), barium titanate (BaTiO₃), and magnesium fluoride (MgF₂).

In any embodiments of the present application, the separator has an air permeability of 100 s/100 mL-300 s/100 mL; optionally, the separator has an air permeability of 150 s/100 mL-250 s/100 mL.

In any embodiments of the present application, the separator has a transverse tensile strength (Machine Direction, MD) of 1500 kgf/cm²-3000 kgf/cm²; optionally, the separator has a transverse tensile strength of 1800 kgf/cm²-2500 kgf/cm².

In any embodiments of the present application, the separator has a longitudinal tensile strength (Transverse Direction, TD) of 1000 kgf/cm²-2500 kgf/cm² ; optionally, the separator has a longitudinal tensile strength of 1400 kgf/cm²-2000 kgf/cm².

In any embodiments of the present application, the separator has a transverse elongation at break of 50%-200%; optionally, the separator has a transverse elongation at break of 100%-150%.

In any embodiments of the present application, the separator has a longitudinal elongation at break of 50%-200%; optionally, the separator has a longitudinal elongation at break of 100%-150%.

In any embodiment of the present application, the inorganic particles and the organic particles form a non-uniform pore structure in the coating.

In any embodiment of the present application, the spacing between any two adjacent inorganic particles is denoted as L1, and the spacing between any inorganic particle and an adjacent organic particle is denoted as L2, wherein L1<L2.

In a second aspect, the present application provides a method for preparing a separator, comprising the steps of: (1) providing a substrate; (2) providing a coating slurry, comprising a component material and a solvent, wherein the component material comprises inorganic particles and organic particles, and the organic particles comprise first organic particles and second organic particles; and (3) coating at least one side of the substrate from step (1) with the coating slurry from step (2), so as to form a coating, and drying same to obtain the separator; wherein the dried coating comprises the inorganic particles, the first organic particles and the second organic particles; the first organic particles and second organic particles are embedded in the inorganic particles and form protrusions on the surface of the dried coating. The first organic particles are secondary particles, and the number of the first organic particles in the dried coating is denoted as A; the second organic particles are primary particles, and the number of the second organic particles in the dried coating is denoted as B; with the separator satisfying: 0.05≤A/B<1.

In any embodiment of the present application, in step (2), the first organic particles are added in a mass percentage of 12% or more of the total dry weight of the component material, optionally 12%-30%.

In any embodiments of the present application, in step (2), the second organic particles is added in a mass percentage of 10% or less of the total dry weight of the component material, optionally 2%-10%.

In any embodiment of the present application, in step (2), the coating slurry has a solid content of 28%-45%, optionally 30%-38%.

In any embodiment of the present application, in step (3), the coating is carried out using a coating machine, wherein the coating machine comprises a gravure roller which has a number of lines of 100 LPI-300 LPI, optionally 125 LPI-190 LPI.

In any embodiment of the present application, in step (3), the coating is carried out at a speed of 30 m/min-90 m/min, optionally 50 m/min-70 m/min.

In any embodiment of the present application, in step (3), the coating is carried out at a line speed ratio of 0.8-2.5, optionally 0.8-1.5.

In any embodiment of the present application, in step (3), the drying is carried out at a temperature of 40° C. to 70° C., optionally 50° C. to 60° C.

In any embodiment of the present application, in step (3), the drying is carried out for a period of 10 s-120 s, optionally 20 s-80 s.

In a third aspect, the present application provides a secondary battery, comprising a separator according to the first aspect of the present application or a separator prepared by the method according to the second aspect of the present application.

In a fourth aspect, the present application provides a battery module, comprising a secondary battery according to the third aspect of the present application.

In a fifth aspect, the present application provides a battery pack, comprising a battery module according to the fourth aspect of the present application.

In a sixth aspect, the present application provides a device, comprising at least one of a secondary battery according to the third aspect of the present application, a battery, module according to the fourth aspect of the present application, or a battery pack according to the fifth aspect of the present application.

The device of the present application comprises at lease one of a secondary battery, a battery module or a battery pack provided by the present application, and thus has at least the same advantages as the secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to illustrate the technical solutions of the present application more clearly, the drawings used in the present application will be described briefly below. Apparently, the drawings described below are merely some embodiments of the present application, and those person of ordinary skill in the art may derive other drawings from these drawings without creative efforts.

FIG. 1 is a structural schematic diagram of an embodiment of a separator of the present application.

FIG. 2-1 is a scanning electron microscope (SEM) picture at a 500×magnification of an embodiment of a separator of the present application.

FIG. 2-2 is a scanning electron microscope (SEM) picture at a 3000×magnification of an embodiment of a separator of the present application.

FIG. 3 is an ion-polished cross-sectional topography (cross-section polishing, CP) picture at a 3000×magnification in an embodiment of a separator of the present application.

FIG. 4-1 is a structural schematic diagram of an embodiment of a separator of the present application,

FIG. 4-2 is a structural schematic diagram of a further embodiment of a separator of the present application.

FIG. 5 is a schematic diagram of an embodiment of a secondary battery.

FIG. 6 is an exploded view of FIG. 5 .

FIG. 7 is a schematic diagram of an embodiment of a battery module.

FIG. 8 is a schematic diagram of an embodiment of a battery pack.

FIG. 9 is an exploded view of

FIG. 10 is a schematic diagram of an embodiment of an device with a secondary battery as a power source.

DETAILED DESCRIPTION OF EMBODIMENTS

The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are merely, intended to illustrate the present application but not to limit the scope of the present application.

For the sake of brevity, only certain numerical ranges are specifically disclosed herein. However, any lower limit may be combined with any upper limit to form a range that is not explicitly described; and any lower limit may be combined with any other lower limit to form an unspecified range, and any upper limit likewise may be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value itself may serve as a lower or upper limit in combination with any other point or single value or with other lower or upper limit to form an unspecified range.

In the description herein, it should he noted that, unless otherwise stated; the recitation of numerical ranges by “no less than” and “no more than” include all numbers within that range including the endpoints, the recitation of “more” in the phrase “one or more” comprises two or more,

In the description herein, unless otherwise stated, the term “or” is inclusive. That is to say, the phrase “A or B” means “A, B, or both A and B.” More specifically, a condition “A or B” is satisfied by any one of the following: A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present).

Unless otherwise stated, the terms used in the present application have the meaning well-known to a person of ordinary skill in the art. Unless otherwise stated, the values of the parameters mentioned in the present application may be measured by various measurement methods commonly used in the art (for example, may be measured according to the method illustrated in the examples of the present application).

Secondary Battery

A secondary battery, refers to a battery which can continue to be used by activating the active material by means of charging after the battery is discharged.

Generally, the secondary battery comprises a positive electrode plate, a negative electrode plate, a separator and an electrolyte. During the charge/discharge process of the battery, active ions are intercalated and de-intercalated back and forth between the positive electrode plate and the negative electrode plate. The separator is provided between the positive electrode plate and the negative electrode plate, and functions for separation. The electrolyte is located between the positive electrode plate and the negative electrode plate and functions for ionic conduction.

[Separator]

The separator provided in the present application comprises: a substrate and a coating provided on at least one surface of the substrate. The coating comprises inorganic particles and organic particles; and the organic particles comprise first organic particles and second organic particles, the first organic particles and second organic particles are embedded in the inorganic particles and form protrusions on the surface of the coating; the first organic particles are secondary particles, and the number of the first organic particles in the coating is denoted as A; the second organic particles are primary particles and the number of the second organic particles in the coating is denoted as B; with the separator satisfying: 0.05≤A/B<1.

The separator of the present application comprises inorganic particles, specific first organic particles and second organic particles in the same coating, such that the rate performance, high-temperature storage performance and puncture safety performance of the battery can be further improved, while ensuring that the battery has a higher energy density.

It should be noted that the primary particles and secondary particles have meanings well-known in the art. A primary particle refers to a particle that does not form an agglomerated state. A secondary particle refers to a particle in an agglomerated state formed by the aggregation of two or more primary particles.

Without wishing to be bound by any theory, it is found through a lot of studies by the inventors that the separator of the present application comprises inorganic particles and organic particles in the same coating, which can greatly reduce the thickness of the separator, compared to a separator comprising two coatings, i.e., an inorganic particles layer and an organic particles layer, thus improving the energy density of the battery; meanwhile, the organic particles comprise first organic particles and second organic particles, and the first organic particles and second organic particles are specially designed in terms of the morphology and number ratio; on the one hand, the first organic particles are secondary particles, which facilitates the formation of an uniform coating interface, and can effectively improve the tabs dislocation problem during the preparation of the battery, thereby improving the safety performance of the battery, the second organic particles are primary particles and thus a large-area adhesive film structure does not easily form between the particles, thereby further improving the rate performance and safety performance of the battery; on the other hand, the use of the two types of particles in combination can effectively improve the infiltration and distribution uniformity of the electrolyte, thereby further improving the high-temperature storage performance of the battery; particularly, when the battery is punctured by a foreign material during the use, the second organic particles can instantly wrap the foreign material and bare copper or aluminum foil to form a first polymer insulation layer, which effectively reduces the probability of short circuit occurred between the positive electrode and negative electrode and improve the safety performance of the battery; as the temperature around the foreign material increases, the first organic particles will form a stronger second polymer insulation layer around the first polymer insulation layer, thereby further improving the safety performance of the battery.

When the A/B is too small (for example, less than 0.05), it may be a case that the first organic particles are too less or the second organic particles are too much; when the first organic particles are too less, the separator will not easily form a strong second polymer insulation layer when the battery is in a harsh use environment, thereby affecting the safety performance of the battery; when the second organic particles are too much, an over-bonding may occur between the separator and the electrode plate, resulting in excessive consumption of the electrolyte, and the decrease of the infiltration and uniformity of the electrolyte, thus affecting the storage performance of the battery.

When the A/B is too large (for example, more than 1), it may be a case that the first organic particles are too much or the second organic particles are too less; when the first organic particles are too much, this will result in a too long ion transmission path, thereby affecting the rate performance of the battery; when the second organic particles are too less, the separator will not easily instantaneously form a first polymer insulation layer when the battery is in a harsh use environment, which greatly increases the risk of a battery short circuit, thereby affecting the safety performance of the battery.

By the intensive study, the present inventors have found that on the basis that the separator of the present application satisfies the design conditions, if one or more of the following conditions are also optionally satisfied, the performance of the secondary battery can be further improved.

In some embodiments, the separator can satisfy: 0.05≤A/B≤0.95, 0.1≤A/B<1, 0.15≤A/B<1, 0.15≤A/B≤0.8, 0.2≤A/B≤0.85, 0.2≤A/B≤0.6, 0.25≤A/B≤0.5, 0.3≤A/B ≤0.75, 0.3≤A/B≤0.6, 0.3≤A/B≤0.5, 0.3≤A/B ≤0.45. When the A/B is within the given range, the rate performance, storage performance and safety performance of the battery can be further improved.

As shown in FIG. 1 , the separator comprises a substrate (A) and a coating (B), wherein the coating (B) comprises first organic particles (B1), second organic particles (B2) and inorganic particles (B3), the first organic particles (B1) are secondary particles, the second organic particles (B2) are primary particles, and the first organic particles and second organic particles are both embedded in the inorganic particles (B3) and form protrusions on the surface of the coating (B).

In some embodiments, the first organic particles have a number-average particle size ≥8 μM; for example, the first organic particles can have a number-average particle size of 10 μm-20 μm, 10.5 μm-18 μm, 12 μm-25 μm, 12 μm-18 μm, 13 μm-20 μm, 15 μm-22 μm. When the number-average particle size of the first organic particles is within the given range, the organic particles are enabled to have sufficient voids therebetween; even though the organic particles swell in the electrolyte, sufficient ion transmission channels can be formed, thereby further improving the rate performance of the battery.

In some embodiments, the second organic particles have a number-average particle size ≥2 μm; for example, the second organic particles can have a number-average particle size of 3 μm-8 μm, 2.5 μm-7 μm, 3 μm-7 μm, 3 μm-6 μm, 3.5 μm-6 μm. When the number-average particle size of the second organic particles is within the given range, the rate performance and safety performance of the battery can be further improved.

In some embodiments, the ratio of the number-average particle size of the first organic particles to that of the second organic particles is ≥2.0 μm; for example, the ratio of the number-average particle size of the first organic particles to that of the second organic particles is 2.0-5.0, 2.0-4.5, 2.0-4.0, 2.5-4.0 and 3.0-4.0. When the ratio of the number-average particle size of the first organic particles to that of the second organic particles is within the given range, the rate performance and safety performance of the battery can be further improved.

It should be noted that the number-average particle size of the organic particles refers to the arithmetic mean of the particle size of the organic particles counted according to the number of the organic particles in the coating of the separator. The particle size of the organic particle refers to the distance between two points on the organic particles that are farthest apart.

In some embodiments, the first organic particles cam comprise one or more of a homopolymer or copolymer of a fluorine-containing olefine monomeric unit, a homopolymer or copolymer of an olefine monomeric unit, a homopolymer or copolymer of an unsaturated nitrile monomeric unit, a homopolymer or copolymer of an alkylene oxide monomeric unit, and modified compounds of these homopolymers or copolymers.

In some embodiments, the fluorine-containing olefine monomeric unit can be selected from one or more of difluoroethylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene, and hexafluoropropylene.

In some embodiments, the olefine monomeric unit can be selected from one or more of ethylene, propylene, butadiene, isoprene.

In some embodiments, the unsaturated nitrile monomeric unit can be selected from one or more of acrylonitrile, methylacrylonitrile.

In some embodiments, the alkylene oxide monomeric unit can be selected from one or more of ethylene oxide, propylene oxide.

In some embodiments, the first organic particles can comprise one or more of polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, polyethylene, polypropylene, polyacrylonitrile, polyethylene oxide, a copolymer of different fluorine-containing olefine monomeric units, a copolymer of a fluorine-containing olefine monomeric unit and an olefine monomeric unit, a copolymer of a fluorine-containing olefine monomeric unit and an acrylic monomeric unit, a copolymer of a fluorine-containing olefine monomeric unit and an acrylate monomeric unit, and modified compounds of these homopolymers or copolymers.

In some embodiments, the first organic particles can comprise one or more of a vinylidene fluoride-trifluoroethylene copolymer, a vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene fluoride-trifluoroethylene-hexafluoropropylene copolymer, a vinylidene fluoride-hexafluoropropylene-acrylic acid copolymer, a vinylidene fluoride-hexafluoropropylene-acrylate copolymer, and modified compounds of these copolymers.

In some embodiments, the second organic particles can comprise one or more of a homopolymer or copolymer of an acrylate monomeric unit, a homopolymer or copolymer of an acrylic monomeric unit, a homopolymer or copolymer of a styrene monomeric unit, a polyurethane compound, a rubber compound, and modified compounds of these homopolymers or copolymers.

In some embodiments, the second organic particles can comprise one or more of a copolymer of an acrylate monomeric unit and a styrene monomeric unit, a copolymer of an acrylic monomeric unit and a styrene monomeric unit, a copolymer of an acrylic monomeric unit, an acrylate monomeric unit and a styrene monomeric unit, a copolymer of a styrene monomeric unit and an unsaturated nitrile monomeric unit, a copolymer of a styrene monomeric unit, an olefine monomeric unit and an unsaturated nitrile monomeric unit, and modified compounds of these copolymer.

In some embodiments, the acrylate monomeric unit can be selected from one or more of methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, butyl rnethacryl ate, isooctyl methacrylate.

In some embodiments, the acrylic monomeric unit can be selected from one or more of acrylic acid, methacrylic acid.

In some embodiments, the styrene monomeric unit can be selected from one or more of styrene, methylstyrene.

In some embodiments, the unsaturated nitrile monomeric unit can be selected from one or more of acrylonitrile, methylacrylonitrile.

In some embodiments, the second organic particles can comprise one or more of a butyl acrylate-styrene copolymer, a butyl methacrylate-isooctyl methacrylate copolymer, an isooctyl methacrylate-styrene copolymer, a methacrylate-methacrylic acid-styrene copolymer, a methyl acrylate-isooctyl methacrylate-styrene copolymer, a butyl acrylate-isooctyl acrylate-styrene copolymer, a butyl acrylate-isooctyl methacrylate-styrene copolymer, a butyl methacrylate-isooctyl acrylate-styrene copolymer, a butyl methacrylate-isooctyl methacrylate-styrene copolymer, a styrene-acrylonitrile copolymer, a styrene-butadiene-acrylonitrile copolymer, a methyl acrylate-styrene-acrylonitrile copolymer, an isooctyl methacrylate-styrene-acrylonitrile copolymer, a styrene-vinyl acetate copolymer, a styrene-vinyl acetate-pyrrolidone copolymer, and modified compounds of these copolymers.

A modified compound of the homopolymer or copolymer refers to a modified compound obtained by copolymerization of the monomeric units in the homopolymer or copolymer with monomeric units containing a specific functional group. For example, a fluorine-containing olefine monomeric unit can be copolymerized with a carboxyl functional group-containing compound to obtain a modified compound thereof, etc.

In some embodiments, the first organic particles have a number-average molecular weight of 300000-800000, for example, 400000-650000.

In some embodiments, the second organic particles have a number average molecular weight of 10000-100000, for example, 20000-80000.

In some embodiments, the inorganic particles can comprise one or more of boehmite (γ-AlOOH), aluminum oxide (Al₂O₃), barium sulfate (BaSO₄), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)₂), silicon dioxide (SiO₂). tin dioxide (SnO₂), titanium oxide (TiO₂), calcium oxide (CaO), zinc oxide (ZnO), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), nickel oxide (NiO), cerium oxide (CeO₂), zirconium titanate (SrTiO₃), barium titanate (BaTiO₃), and magnesium fluoride (MgF₂); for example, the inorganic particles can comprise one or more of boehmite (γ-AlOOH), and aluminum oxide (Al₂O₃).

In some embodiments, the inorganic particles have a volume-average particle size Dv50 of ≤2.5 for example, the inorganic particles can have a volume-average particle size Dv50 of 0.5 μm-2.5 μm and 0.5 μm-1 μm. When the volume-average particle size of the inorganic particles is controlled within the given range, the volume energy density of the battery can be further improved while ensuring better cycling performance and safety performance of the battery.

In some embodiments, the mass percentage of the inorganic particles in the coating ≤75% (based on the total mass of the coating); for example, the mass percentage of the inorganic particles in the coating can be 60%-75%, 60%-70% and 65%-75%. When the mass percentage of the inorganic particles is controlled within the given range, the mass energy density of the battery can be further improved while ensuring the good safety, performance of the battery.

In some embodiments, the mass percentage of the first organic particles in the coating is ≥12% (based on the total mass of the coating); for example, the mass percentage of the first organic particles in the coating can be 12%-30%, 15%-30%, 15%-25%, 15%-20%, 16%-18%. When the mass percentage of the first organic particles is within the given range, the above A/B range of the present application can be more easily achieved; furthermore; a suitable mass percentage range also can reduce the consumption of the electrolyte by the separator, thereby further improving the storage performance and safety performance of the battery.

In some embodiments, the mass percentage of the second organic particles in the coating is ≤10% (based on the total mass of the coating); for example, the mass percentage of the second organic particles in the coating can be 2%-10%, 3%-7% and 3%-5%. When the mass percentage of the second organic particles is controlled within the given range, the above A/B range of the present application can be more easily achieved, it is helpful for the separator to have a suitable pore structure while ensuring a bonding property, thereby further improving the rate performance and safety performance of the battery.

By selecting suitable contents of the inorganic particle, the first organic particles and the second organic particles, a better synergistic effect of the three types of particles can be achieved, ensuring that the separator further has a suitable and non-uniform pore structure while ensuring the safety performance, and at the same time achieves a light-weight separator, thereby further improving the energy density of the battery.

In some embodiments, the separator has a single-sided coating weight per unit area of ≤3.0 g/m²; for example, the separator can have a single-sided coating weight per unit area of 1.5 g/m²-3.0 g/m², 1.5 g/m²-2.5 g/m², 1.8 g/m²-2.3 g/m². When the weight of single-sided coating on the separator per unit area is controlled within the given range, the rate performance and safety performance of the battery can be further achieved while increasing the energy density of the battery.

In some embodiments, the coating can further comprise other organic compounds, for example, a polymer that improves the heat resistance (referred to as “heat-resistant adhesive” for short), a dispersant, a wetting agent, other types of bonders, etc. The above other organic compounds are all non-granular substances in the coating. In the present application, the above other organic compounds are not particularly limited in types, and can be selected from any well-known materials with well improved performance.

In an embodiment of the present application, the material of the substrate is not particularly limited, and can be selected from any well-known substrate with good chemical and mechanical stability, for example one or more of glass fibers, a non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The substrate can be a single-layer film, or a multi-layer composite film. When the substrate is a multi-layer composite film, the materials of the respective layers may be the same or different.

In some embodiments, the substrate has a thickness of ≤10 μm; for example, the substrate can have a thickness of 5 μm-10 μm, 5 μm-9 μm, and 7 μm-9 μm. When the thickness of the substrate is controlled within the given range, the energy density of the battery can be further improved while ensuring the rate performance and safety performance of the battery.

In some embodiments, the separator can have an air permeability of 100 s/100 mL-300 s/100 mL; for example, the separator can have an air permeability of 150 s/100 mL-250 s/100 mL, 170 s/100 mL-220 s/100 mL.

In some embodiments, the separator can have a longitudinal tensile strength (TD) of 1000 kgf/cm²-2500 kgf/cm²; for example, the separator can have a longitudinal tensile strength of 1400 kgf/cm²-2000 kgf/cm².

In some embodiments, the separator can have a longitudinal elongation at break of 50%-200% for example, the separator can have a longitudinal elongation at break of 100%-150%.

In some embodiments, the separator can have a transverse tensile strength (MD) of 1500 kgf/cm²-3000 kgf/cm²; for example, the separator can have a transverse tensile strength of 1800 kgf/cm²-2500 kgf/cm².

In some embodiments, the separator can have a transverse elongation at break of 50%-200%; for example, the separator can have a transverse elongation at break of 100%-150%.

In some embodiments, the spacing between any two adjacent inorganic particles is denoted as L1, and the spacing between any inorganic particle and an adjacent organic particle is denoted as L2, wherein L1<L2.

According to some embodiments, the ratio of the number of the first organic particles to that of the second organic particles can be determined by a method known in the art, for example, by a scanning electron microscope. As an example, the ratio of the number of the first organic particles to that of the second organic particles can be determined by a method: a separator is made into a test sample with length×width=50 mm×100 mm; the organic particles in the coating are measured using a scanning electron microscope (for example, ZEISS Sigma300). The test can be carried out by referring to JY/T010-1996. A plurality of (for example, 5) different areas are randomly selected from the test sample and scanned; under a magnification (for example, 500× or 1000×), the numbers of the first organic particles or second organic particles in the test areas are recorded; an average value of the numbers of the first organic particles in the areas are taken and denoted as A, and an average value of the numbers of the second organic particles in the areas are taken and denoted as B; the value of A/B is calculated. In order to ensure the accuracy of the test results, a plurality of (for example, 10) test samples can be taken for the repetition of the measurement, and an average value of A/B is calculated from the test samples, as the final measurement result.

FIG. 2-1 is a scanning electron microscope (SEM) image at a 500×magnification of a separator in an embodiment of the present application. In FIG. 2-1 , the number A of the first organic particles in the coating and the number B of the second organic particles in the coating can be recorded according to the method given above.

According to some embodiments, the spacing between any two adjacent inorganic particles is determined by randomly selecting two inorganic particles in the coating (when the inorganic particles are of an irregular shape, the particles can be circumscribed to form a circle) in the SEM image of the separator, and measuring the spacing between the centers of circles of the two inorganic particles as the spacing between the two inorganic particles, denoted as L1.

According to some embodiments, the spacing between any inorganic particle and an adjacent organic particle is determined by randomly selecting an inorganic particle and an organic particle in the coating (when the inorganic particle or organic particle are of an irregular shape, the particle can be circumscribed to form a circle) in the SEM image of the separator, and measuring the spacing between the centers of circles of the inorganic particle and the organic particle as the spacing between the inorganic particle and the organic particle, denoted as L2. The mentioned organic particle may be a first organic particle, or a second organic particle.

The spacing can be determined using an instrument known in the art. For example, it can be determined by a scanning electron microscope. As an example, the spacing L2 between any inorganic particle and an adjacent organic particle can be measured as follows: a separator is made into a test sample with length×width=50 mm×100 mm; the separator is measured using a scanning electron microscope (for example, ZEISS Sigma300). The measurement can be carried out by referring to JY/T010-1996, An area is randomly selected in the test sample for scanning, to obtain an SEM image of the separator under a certain magnification (for example, 3000×); in the SEM image, an inorganic particle and an adjacent organic particle are randomly selected (when the inorganic particle or organic particle is an irregular body, the particle can be circumscribed to form a circle), to measure the distance between the centers of circles of the inorganic particle (or the circumscribed circle thereof) and the organic particle (or the circumscribed circle thereof), as the spacing between the inorganic particle and adjacent organic particle of the present application, denoted as L2, In order to ensure the accuracy of the measurement results, a number of groups of adjacent particles (for example, 10 groups) can be selected in the test sample to repeat the measurement, and an average of the test results on the groups are taken as the final result.

Similarly, the spacing between any two adjacent inorganic particles L1 can also be measured according to the above method.

FIG. 2-2 is a scanning electron microscope (SEM) image at a 3000×magnification of a separator in an embodiment of the present application, It can be seen from FIG. 2-2 that the coating of the separator comprises inorganic particles, first organic particles and second organic particles, wherein the first organic particles are secondary particles, the second organic particles are primary particles, and the first organic particles and second organic particles are embedded in the inorganic particles and form protrusions on the surface of the coating. By the measurement according to the method described above, it can be derived that L1<L2.

According to some embodiments, the morphology of the organic particles (for example: primary particle morphology or secondary particle morphology) can be determined by an apparatus and method known in the art. For example, a scanning electron microscope (for example, ZEISS Sigma 300) can be used for the determination. As an example, the following steps are performed: first, a separator is cut into a sample to be tested with a certain size (for example, 6 mm×6 mm), and the sample to be tested is sandwiched by two electrically and thermally conductive sheets (for example, copper foils), and the sample to be tested is sticked and fixed to the sheets by an adhesive (for example, a double-sided adhesive tape), and pressed with a flat iron block having a mass (for example, 400 g) for a period of time (for example, 1 h), such that the gaps between the sample to be tested and the copper foils are as small as possible, the edges are then trimmed using scissors; the sample to be tested is sticked onto a sample stage with a conductive adhesive, with the sample slightly protruding from the edge of the sample stage. Then, the sample stage is mounted onto a sample holder and locked for fixation; the power of an argon ion cross section polisher (for example, IB-19500CP) is turned on for vacuumization (for example, 10 Pa-4 Pa); the argon flow rate (for example, 0.15 MPa) and voltage (for example, 8 KV) and polishing time (for example, 2 hours) are set, the sample stage is adjusted to a rocking mode to start the polishing; after the completion of the polishing, the ion-polished cross-sectional topography (CP) picture of the sample to be tested is obtained by using a scanning electron microscope (for example, ZEISS Sigma 300).

FIG. 3 is an ion-polished cross-sectional topography (CP) picture at a 3000×magnification of a separator in embodiments of the present application. It can be seen from FIG. 3 that, the coating of the separator comprises the first organic particles and the second organic particles; The first organic particles are secondary particles composed of a plurality of primary particles and have an irregular non-solid spherical cross-section; The second organic particles are non-agglomerated primary particles and have a solid spherical cross-section.

According to some embodiments, the particle size and number-average particle size of the organic particles can be measured by an apparatus and method known in the art. For example, a scanning electron microscope (for example, ZEISS Sigma 300) is used to obtain an scanning electron microscope (SEM) picture of the separator, for example, by, referring to JY/T010-1996. As an example, the measurement can be carried out as follows: a test sample with length×width=50 mm×100 mm is randomly selected from the separator and a plurality of (for example, 5) test areas are randomly selected on the test sample; at a certain magnification (for example, 500× when measuring the first organic particles and 1000× when measuring the second organic particles), the particle sizes (i.e., the distance between two points that are farthest apart is taken as the particle size of the organic particle) of the respective organic particles in the respective test areas are read. The numbers and particle size values of the organic particles in the respective test areas are recorded, and an arithmetic mean of the particle sizes of the organic particles in the test areas are taken as the number-average particle size of the organic particles in the test sample. In order to ensure the accuracy of the test results, the above measurement can be repeated using a plurality of (for example, 10) test samples, and an average of these test samples is taken as the final measurement result.

According to some embodiments, the material type of the organic particles can be determined by an apparatus and method known in the art. For example, infrared spectrum of the the material can be measured, so as to determine the characteristic peaks contained therein, and thus to determine the material type. Specifically, the organic particles can be analyzed by infrared spectroscopy using instruments and methods known in the art, for example an infrared spectrometer, for example, be determined by an IS10 Fourier transform infrared spectrometer from Nicolet, USA, and according to the GB/T6040-2002 General rules for infrared spectrum analysis.

According to some embodiments, the volume average particle size Dv50 of the inorganic particles has the meaning well-known in the art, and can be determined by the instrument and method known in the art. For example, it can be determined by referring to GB/T 19077-2016 particle size distribution-laser diffraction method, using a laser particle size analyzer (for example, Master Size 3000).

According to some embodiments, the air permeability, transverse tensile strength (MD), longitudinal tensile strength (TD), transverse elongation at break, and longitudinal elongation at break of the separator all have meanings well-known in the art, and can be determined according to the methods known in the art. For example, they can all be determined by referring to GB/T 36363-2018.

The present application also provides a method for preparing the separator, comprising the steps of:

-   -   (1) providing a substrate;     -   (2) providing a coating slurry: the coating slurry comprising a         component material and a. solvent, wherein the component         material comprises inorganic particles and organic particles,         and the organic particles comprise first organic particles and         second organic particles; and     -   (3) coating at least one side of the substrate from step (1)         with the coating slurry from step (2) so as to form a coating,         and drying same to obtain the separator;         wherein the dried coating comprises the inorganic particles, the         first organic particles and the second organic particles; the         first organic particles and second organic particles are         embedded in the inorganic particles and form protrusions on the         surface of the dried coating;         the first organic particles are secondary particles, and the         number of the first organic particles in the dried coating is         denoted as A; the second organic particles are primary particles         and the number of the second organic particles in the dried         coating is denoted as B; with the separator satisfying:         0.05≤A/B<1.

As shown in FIG. 4-1 , the separator comprises a substrate (A) and a coating (B), and the coating (B) is provided on only one surface of the substrate (A).

As shown in FIG. 4-2 , the separator comprises a substrate (A) and a coating (B), and the coating (B) is provided on both surfaces of the substrate (A) at the same time.

In an embodiment of the present application, the material of the substrate is not particularly limited, and can be selected from any well-known substrate with good chemical and mechanical stability, for example one or more of glass fibers, a non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The substrate can be a single-layer film, or a multi-layer composite film. When the substrate is a multi-layer composite film, the materials of the respective layers may be the same or different.

In some embodiments, in step (2), the solvent can be water, for example, deionized water.

In some embodiments, in step (2), the component material also can comprise other organic compounds, for example, a polymer that improve the heat resistance, a dispersant, a wetting agent, other types of bonders. In such a case, the other organic compounds are all non-granular in the dried coating.

In some embodiments, in step (2), a component material is added to the solvent and stirred uniformly to obtain a coating slurry.

In some embodiments, in step (2), the first organic particles are added in a mass percentage of 12% or more of the total dry weight of the component material; for example, 12%-30%, 15%-30%, 15%-25%, 15%-20% and 16%-22%.

In some embodiments, in step (2), the second organic particles are added in a mass percentage of 10% or less of the total dry weight of the component material, for example, 2%-10%, 3%-7% and 3%.-5%.

When the first organic particles or the second organic particles are added in a mass within the above range, it is helpful to control the ratio of the numbers of the first organic particles and the second organic particles within the given range. meanwhile, an appropriate content of the organic particles can reduce the static electricity generated between the separator and a battery winding tool (such as, a rolling pin) or laminating tool during the preparation of the battery, and effeaively reduce the probability of short circuit between the positive electrode and negative electrode, thereby improving the manufacturing yield of the battery.

It should be noted that, when the component material is in a solid state, the dry weight of the component material is the mass of the component material that is added. When the component material is in a suspension, an emulsion or a solution, the dry weight of the component material is a product of the mass of the component material that is added and the solid content of the component material. The total dry weight of the component material ingredients is the sum of the dry weights of the component material ingredients.

In some embodiments, in step (2), the solid content of the coating slurry can be controlled at 28%-45%, for example, 30%-38%. When the solid content of the coating slurry is within the above range, the film surface problem ( ) of the coating can be effectively reduced and the probability of non-uniform coating can be reduced, thereby further improving the cycling performance and safety performance of the battery.

In some embodiments, in step (3), the coating is carried out by a coating, machine.

In an embodiment of the present application, the model of the coating machine is not particularly limited, and a commercially available coating machine can be used.

In some embodiments, in step (3), the coating can be carried out by a process, such as transfer coating, rotary spraying, dip coating, etc.; for example, the coating is carried out by transfer coating.

In some embodiments, the coating machine comprises a gravure roll; and the gravure roller is used for transferring the coating slurry to the substrate.

In some embodiments, the gravure roller can have a number of lines of 100 LPI-300 LPI, for example, 125 LPI-190 LPI (LPI represents lines/inch). When the number of lines of the gravure roller is within the above range, it is helpful to control the ratio of the number of the first organic particles to that of the second organic particles within the given range, thereby further improving the cycling performance and safety performance of the separator.

In some embodiments, in step (3), the speed for coating can be controlled at 30 m/min-90 m/min, for example. 50 m/min-70 m/min. When the coating is carried out at a speed within the above range, it is helpful to control the ratio of the number of the first organic particles to that of the second organic particles within the given range; moreover, the film surface problem of the coating can be effectively reduced, and the probability of non-uniform coating can be reduced, thereby further improving the cycling performance and safety performance of the battery.

In some embodiments, in step (3), the line speed ratio for coating can be controlled at 0.8-2.5, for example, 0.8-1.5, and 1.0-1.5.

In some embodiments, in step (3), the drying can be carried out at a temperature of 40° C.-70° C., for example, 50° C. to 60° C.

In some embodiments, in step (3), the drying can be carried out for a period of 10 s-120 s, for example, 20 s-80 s, and 20 s-40 s.

By controlling the above process parameters within the given ranges, the operational performance of the separator in the present application can be further improved. Those of ordinary skill in the art can selectively adjust and control one or more of the above process parameters according to the actual production.

In order to further improve the performance of the secondary battery, the inorganic particles and the organic particles also optionally satisfy one or more of the aforementioned parameter conditions. It will not be repeated here.

The above substrate, first organic particles and second organic particles are all commercially available.

In the method for preparing the separator of the present application, the coating is prepared by one-time coating, which greatly simplifies the production process for a separator; Meanwhile, the use of the separator prepared by the above method in a battery can effectively improve the rate performance, storage performance and safety performance of the battery.

[Positive Electrode Plate]

In a secondary battery, a positive electrode plate comprises a positive electrode current collector and a positive electrode film layer provided on the positive electrode current collector, wherein the positive electrode film layer comprises a positive electrode active material.

The positive electrode current collector may be a conventional metal foil or a composite current collector (for example, a composite current collector can be formed by providing a metal material on a polymer substrate). As an example, the positive electrode current collector may be an aluminum foil.

The specific types of the positive electrode active materials are not limited, and active materials known in the art that can be used for the positive electrode of secondary batteries can be used, and the active materials can be selected by those skilled in the art according to actual requirements.

As an example, the positive electrode active material can include, but is not limited to, one or more of lithium transition metal oxides, lithium-containing phosphates with a olivine structure and the respective modified compounds thereof. An example of the lithium transition metal oxide can include, but is not limited to, one or more of lithium cobalt oxides, lithium nickel oxides, lithium manganese oxides, lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, lithium nickel cobalt manganese oxides, lithium nickel cobalt aluminum oxides and the respective modified compounds thereof. An example of the lithium-containing phosphates with a olivine structure can include, but is not limited to, one or more of lithium iron phosphate, a lithium iron phosphate-carbon composite, lithium manganese phosphate, a lithium manganese phosphate-carbon composite, lithium iron manganese phosphate, a lithium iron manganese phosphate-carbon composite and modified compounds thereof. These materials are all commercially available.

In some embodiments, the modified compounds of these material can be from the doping modification and/or surface coating modification of the material.

The positive electrode film layer typically also optionally comprises a bonder, a conductive agent and other optional auxiliary agents.

As an example, the conductive agent can be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, Super P (SP), graphene and carbon nanofibers.

As an example, the bonder can be one or more of a styrene-butadiene rubber (SBR), a water-based acrylic resin (water-based acrylic resin), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB).

[Negative Electrode Plate]

In a secondary battery, the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer provided on the negative electrode current collector, wherein the negative electrode film layer comprises a negative electrode active material.

The negative electrode current collector may be a conventional metal foil or a composite current collector (for example, a composite current collector can be formed by providing a metal material on a polymer substrate). As an example, the negative electrode current collector may be a copper foil.

The specific types of the negative electrode active materials are not limited, and active materials known in the art that can be used for the negative electrode of secondary batteries can be used, and the active materials can be selected by a person skilled in the art according to actual requirements. As an example, the negative electrode active mated al can include, but is not limited to, one or more of synthetic graphite, natural graphite, hard carbon, soft carbon, a silicon-based material and a tin-based material, The silicon-based material can be selected from one or more of elemental silicon, a silicon oxide compound (for example, silicon(II) oxide), a silicon-carbon composite, a silicon-nitrogen composite, and a silicon alloy. The tin-based material can be selected from one or more of elemental tin, a tin oxide compound, and a tin alloy. These materials are all commercially available.

In some embodiments, in order to further improve the energy density of the battery, the negative electrode active material can comprise a silicon-based material.

The negative electrode film layer generally also optionally comprises a bonder, a conductive agent and other optional auxiliary agents.

As an example, the conductive agent can be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

As an example, the bonder can be one or more of a styrene-butadiene rubber (SBR), a water-based acrylic resin (water-based acrylic resin), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), an ethylene-vinyl acetate copolymer (EVA), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB).

As an example, other optional auxiliary agents may be a thickening and dispersing agent (for example, sodium carboxymethyl cellulose, CMC-Na), a PTC thermistor material.

[Electrolyte]

The secondary battery can comprises an electrolyte, wherein the electrolyte is between the positive electrode and the negative electrode and functions for ionic conduction. The electrolyte can comprise an electrolyte salt and a solvent.

As an example, the electrolyte salt can be selected from one or more of LiPF6 (lithium hexafluorophosphate), LiBF4(lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexalluoroarsenate), LiFSI (lithium bisfluorosulfonimide), LiTFSI (lithium bistrifluoromethanesulfonimide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate). LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium bisoxalatodifluorophosphate) and LiTFOP (lithium tetrafluorooxalate phosphate).

As an example, the solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS) and diethyl sulfone (ESE).

In some embodiments, the electrolyte also comprises an additive, For example, the additive can include a negative electrode film-forming, additive, a positive electrode film-forming additive, and also an additive that can improve certain performance of the battery, such as an additive to improve the overcharge performance of a battery, an additive to improve the high temperature performance of a battery, and an additive to improve the low temperature performance of a battery, etc.

In some embodiments, the secondary battery can be a lithium-ion secondary battery.

In embodiments of the present application, the shape of the secondary battery is not particularly limited, which can be a cylindrical shape, a prismatic shape or any other shapes. FIG. 5 shows a prismatic secondary battery 5 as an example.

In some embodiments, the secondary battery can comprises an outer package. The outer package is used for packaging the positive electrode plate, the negative electrode plate and the electrolyte.

In some embodiments, referring to FIG. 6 , the outer package can include a housing 51 and a cover plate 53, wherein the housing 51 can include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose and form an accommodating cavity. The housing 51 has an opening in communication with the accommodating cavity, and the cover plate 53 can cover the opening to close the accommodating cavity.

The positive electrode plate, the negative electrode plate and the separator can form an electrode assembly 52 by a winding process or a lamination process. The electrode assembly 52 is packaged in the accommodating cavity. The electrolyte is infiltrate into the electrode assembly 52. The number of the electrode assemblies 52 contained in the secondary battery 5 may be one or more, and may be adjusted according to requirements.

In some embodiments, the outer package of the secondary battery can be a hard shell, for example, a hard plastic shell, an aluminum shell, a steel shell. The outer package of the secondary battery can also be a soft bag, such as a pouch-type soft bag. The material of the soft bag can be a plastic, for example, comprising one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS).

In some embodiments, the secondary battery can be assembled into a battery module, and the number of the secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

FIG. 7 shows a battery module 4 as an example. Referring to FIG. 7 , in the battery module 4, a plurality of secondary batteries 5 can be provided sequentially in the length direction of the battery module 4. Apparently, the secondary batteries can also be disposed in any other manner. Furthermore, the plurality of secondary batteries 5 can be fixed by fasteners.

In some embodiments, the battery module 4 may also comprise a housing with an accommodating space, and a plurality of secondary batteries 5 are accommodated in the accommodating space.

In some embodiments, the above battery module can also be assembled into a battery pack, and the number of the battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

FIG. 8 and FIG. 9 show a battery pack 1 as an example. Referring to FIG. 8 and FIG. 9 , the battery pack 1 can include a battery case and a plurality of battery modules 4 provided in the battery case. The battery case comprises an upper case body 2 and a lower case body 3, wherein the upper case body 2 can cover the lower case body 3 to form a closed space for accommodating the battery modules 4. A plurality of battery modules 4 can be disposed in the battery case in any manner.

Device

The present application further provides a device which comprises at least one of the secondary battery, battery module, or battery pack. The secondary battery, battery module or battery pack may be used as a power source of the device or as an energy storage unit of the device. The device may be, but is not limited to, a mobile device (e.g,, a mobile phone, a laptop computer, etc.), an electric vehicle (e.g., a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck), an electric train, ship, and satellite, an energy storage system, and the like.

The device can incorporate the secondary batten batterymodule or battery pack according to its usage requirements.

FIG. 10 shows a device as an example. The device may be a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle or the like. In order to meet the requirements of the device for a high power and a high energy density of a secondary battery, a battery pack or a battery module can be used.

As another example, the device can be a mobile phone, a tablet computer, a laptop computer. The device is generally required to be thin and light, and may use a secondary battery as a power source.

The beneficial effects of the present application will be further described below in conjunction with embodiments.

EXAMPLES

In order to make the technical problems solved by, the technical solutions and the beneficial effects of the present application clearer, further detailed description will be given below with reference to the embodiments and the accompanying drawings. Apparently, the described embodiments are merely some of, but not all of, the embodiments of the present application. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way to limit the present application and the use thereof. All the other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without any creative effort shall fall within the scope of protection of the present application.

I. Preparation of Separator

separator 1:

-   -   (1) a PE substrate is provided, for example, the substrate         having a thickness of 7 μm, and a porosity of 40%;     -   (2) formulation of coating slurry: inorganic particles of         aluminum oxide (Al₂O₃), first organic particles of a vinylidene         fluoride-hexafluoropropylene copolymer (with a number-average         molecular weight of 550000), second organic particles of a butyl         methacrylate-isooctyl acrylate-styrene copolymer (with a         number-average molecular weight of 80000), a dispersant of         sodium carboxymethyl cellulose (CMC-Na), a wetting agent of an         organosilicon modified poly-ether and a heat-resistent adhesive         of an acrylic acid-acrylonitrile copolymer, are uniformly mixed         at a dry weight ratio of 71:12:10:1.5:0.5:5 in an appropriate         amount of a solvent of deionized water, to obtain a coating         slurry with a solid content of 38% (by weight), wherein the         inorganic particles of aluminum oxide (Al₂O₃) have a         volume-average particle size Dv50 of 15.0 μm, the first organic         particles are secondary particles and have a number-average         particle size of 15.0 μm and the second organic particles are         primary particle and have a number-average particle size of 4.8         μm.     -   (3) the two surfaces of the PE substrate are coated with the         coating slurry formulated in step (2) using a coating machine,         followed by drying and slitting procedures, etc., to obtain         separator 1, wherein the aforementioned coating formulation         process or coating process can be supplementarily adjusted, such         that the ratio A/B of the numbers of the first to the second         organic particles is 0.05. the gravure roller of the coating         machine has a number of lines of 125 LPI, and the coating is         carried out at a speed of 50 m/min, and a line speed ratio of         1.15; the drying is carried out at a temperature of 50° C. for a         period of 25 s; and In the separator, the first organic         particles and the second organic particles are both embedded in         the inorganic particles and form protrusions on the surface of         the coating.

Materials used in the examples are all commercially available, for example: the substrate can be purchased from Shanghai Enjie New Materials Co., Ltd.; the inorganic particles can be purchased from Estone Materials Technology Co Ltd.; the first organic particles can be purchased from Arkema (Changshu) Chemical Co., Ltd.; the second organic particles can be purchased from Sichuan Indile Technology Co., Ltd.; the heat-resistant adhesive can be purchased from Sichuan Indile Technology Co., Ltd.; the wetting agent can be purchased from Dow Chemical Company; and the dispersant can be purchased from Changshu Weiyi Technology Co., Ltd.

The preparation methods for separators 2-40 are similar to that for separator 1, except that: the number-average particle sizes, mass percentages, material types, etc., of the first organic particles and the second organic particles have been adjusted, see Table 1 for details.

II. Preparation of Battery Example 1 1. Preparation of Positive Electrode Plate

A positive electrode active material of LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ (NCM523), a conductive agent of carbon black (Super P), and a binder of polyvinylidene fluoride (PVDF) are uniformly mixed at a mass ratio of 96.2:2.7:1.1 in an appropriate amount of a solvent of N-methyl pyrrolidone (NMP), to obtain a positive electrode slurry; then the positive electrode slurry is coated onto a positive electrode current collector of aluminum foil, followed by drying, cold pressing, slitting and cutting procedures; etc., to obtain a positive electrode plate. The positive electrode has an areal density of 0.207 mg/mm², and a

2. Preparation of Negative Electrode Plate

A negative electrode active material of synthetic graphite, a conductive agent of carbon black (Super P), a bonder of a styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC-Na) at a mass ratio of 96.4:0.7:1.8:1.1 are uniformly mixed in an appropriate amount of a solvent of deionized water to obtain a negative electrode slurry; then the negative electrode slurry is coated onto a negative electrode current collector of copper foil, followed by drying, cold pressing; slitting and cutting procedures, etc., to obtain a negative electrode plate. The negative electrode has an areal density of 0.126 mg/mm2, and a compacted density of 1.7 g/cm³.

3. Separator

Separator 1 prepared as above is used.

4. Preparation of Electrolyte

Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a mass ratio of 30:70, to obtain an organic solvent; then a fully dried electrolyte salt of LiPF₆ is dissolved in the above mixed solvent, with an electrolyte salt concentration of 1.0 mol/L, and uniformly mixed to obtain an electrolyte.

5. Preparation of Secondary Battery

The positive electrode plate, the separator, and the negative electrode plate are laminated in sequence, such that the separator is located between the positive electrode plate and the negative electrode plate and functions for separation, and then they are wound to obtain an electrode assembly; the electrode assembly is placed in an outer package; the electrolyte prepared above is injected into the dried secondary battery, followed by vacuum packaging, leaving to stand, formation, and shaping procedures, etc., to obtain a secondary battery.

The preparation methods for the secondary batteries in examples 2-36 and comparative examples 1-4 are similar to that for the secondary battery in example 1, except that different separators are used, see Table 2 for details.

III. Battery performance test 1. 2C Rate Performance Test

At 25° C.; the secondary batteries in the examples and comparative examples are charged at a constant current rate of 0.33 C to 4.20 V, then charged at a constant voltage to a current of 0.05 C, and left to stand for 5 min; the charge capacity at the moment is recorded, i.e., initial charging capacity; the batteries are left to stand for 5 min, then discharge to 2.8 V at a constant current rate of 0.33 C, and left to stand for 30 min. Then, the secondary batteries are charged at a constant current rate of 2 C to 4.20 V, then charged at a constant voltage to a current of 0.05 C, left to stand for 5 min; the charge capacity at the moment is recorded.

The capacity retention rate at 2C charge rate of the battery (%)=2C charging capacity/0.33 C charging capacity×100%

2. 60° C. High Temperature Storage Performance Test

At 25° C., the secondary batteries prepared in the examples and comparative examples are charged at a rate of 1 C to an end-of-charge voltage of 4.2 V, then charged at a constant voltage to a current of 0.050, left to stand for 5 min, then discharged at a constant current rate of 0.33 C to an end-of-discharge voltage of 2.8 V; the initial capacity of the battery is obtained. Then at 25° C., the battery is charged at a rate of 1 C to an end-of-charge voltage of 4.2 V, and then charged at a constant voltage to a current of 0.05 C; at the moment, the battery is fully charged, and the fully charged battery is placed in a constant temperature box at 60° C. and stored, The battery is taken out every 7 days and discharged at a rate of 0.33 C at 25° C. to an end-of-discharg,e voltage of 2.8 V, left to stand for 5 min, and then charged at 1 C to an end-of-charge voltage of 4.2 V, and then charged at a constant voltage to a current of 0.05 C, left to stand for 5 min, and then discharge at a constant current rate of 0.33 C, and the capacity of the battery at the moment is measured. Until the capacity fades to 80% of the initial capacity, the number of the days of storage is recorded. (It should be notated that every time the battery is taken out and the capacity is measured, the battery needs to be charged at a rate of 1 C to an end-of-charge voltage of 4.2 V, and then charged at a constant voltage to a current of 0.05 C; such that the battery is fully charged, and then placed in a constant temperature box at 60° C. and stored.)

3. Nail Penetration Test

At 25° C., the secondary batteries prepared in the examples and comparative examples (several for each example and comparative example) are charged at a constant current rate of IC to an end-of-charge voltage of 4.2 V, then charged at a constant voltage to a current of ≤0.050, and left to stand for 10 min; the majority of the housing of a battery is peeled off, and the battery is clamped with a fixture, and then penetrated using a 1 mm steel needle at a speed of 0.1 mm/s until a thermal runaway of the battery occurs, and the nail penetration depth at the moment is recorded as L0;

the above operation steps are repeated with a further battery, except that the nail penetration depth is controlled at L0-0.1 mm, and the battery is left to stand and observed for 1 h; if a thermal runaway of the battery occurs during the above observation, the above operation steps are repeated with another battery, except that the nail penetration depth is controlled at L0-0.2 trim; and so on, until no thermal runaway of the battery occurs during the above observation, the nail penetration depth at the moment is recorded as L, and L represents the nail penetration depth under safe use of the battery.

TABLE 1 Number- Number- average average particle particle Mass size of size of ratio First first Second second of Organic organic organic organic materials particle particle particle particle in No. type (μm) type (μm) coating Separator Vinylidene 15.0 Butyl 4.8 71:12:10: 1 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 72:12:9: 2 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 73:12:8: 3 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 75:12:6: 4 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 66:18:9: 5 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 67:18:8: 6 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 68:18:7: 7 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 69:18:6: 8 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 72:18:3: 9 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 65:24:4: 10 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 66:24:3: 11 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 1.0 69:18:6: 12 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 2.0 69:18:6: 13 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 3.0 69:18:6: 14 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 3.5 69:18:6: 15 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.2 69:18:6: 16 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 5.0 69:18:6: 17 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 5.8 69:18:6: 18 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 6.5 69:18:6: 19 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 7.0 69:18:6: 20 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 7.5 69:18:6: 21 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 5.0 Butyl 4.8 69:18:6: 22 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 8.0 Butyl 4.8 69:18:6: 23 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 10.0 Butyl 4.8 69:18:6: 24 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 12.0 Butyl 4.8 69:18:6: 25 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 13.5 Butyl 4.8 69:18:6: 26 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 16.5 Butyl 4.8 69:18:6: 27 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 18.0 Butyl 4.8 69:18:6: 28 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 20.0 Butyl 4.8 69:18:6: 29 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 22.0 Butyl 4.8 69:18:6: 30 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 25.0 Butyl 4.8 69:18:6: 31 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 27.0 Butyl 4.8 69:18:6: 32 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Poly- 15.0 Butyl 4.8 69:18:6: 33 ethylene meth- 1.5:0.5:5 acrylate- isooctyl acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 69:18:6: 34 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene- isooctyl acrylic acid acrylate- copolymer styrene copolymer Separator Vinylidene 15.0 Styrene- 4.8 69:18:6: 35 fluoride- vinyl 1.5:0.5:5 hexafluoro- acetate propylene copolymer copolymer Separator Vinylidene 15.0 Styrene- 4.8 69:18:6: 36 fluoride- vinyl 1.5:0.5:5 hexafluoro- acetate- propylene pyrrolidone copolymer copolymer Separator Vinylidene 15.0 Butyl 4.8 75:6:12: 37 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 Butyl 4.8 61:30:2: 38 fluoride- meth- 1.5:0.5:5 hexafluoro- acrylate- propylene isooctyl copolymer acrylate- styrene copolymer Separator Vinylidene 15.0 / / 75:18:0: 39 fluoride- 1.5:0.5:5 hexafluoro- propylene copolymer Separator / / Butyl 4.8 87:0:6: 40 meth- 1.5:0.5:5 acrylate- isooctyl acrylate- styrene copolymer

Note: Mass ratio of material in coating of inorganic particle: first organic particles:

second organic particle: dispersant: wetting agent: heat-resistant adhesive

TABLE 2 60° C. Rate storage Pene- perfor- perfor- tration mance mance depth Examples Separator A/B (%) (day) (mm) Example 1 Separator 0.05 78 160 5.50 1 Example 2 Separator 0.10 79 165 5.60 2 Example 3 Separator 0.15 81 170 5.80 3 Example 4 Separator 0.20 82 172 5.85 4 Example 5 Separator 0.25 83 175 5.96 5 Example 6 Separator 0.30 84 182 6.17 6 Example 7 Separator 0.40 86 184 6.28 7 Example 8 Separator 0.45 90 185 6.46 8 Example 9 Separator 0.58 85 180 6.15 9 Example 10 Separator 0.80 76 152 5.84 10 Example 11 Separator 0.92 75 145 5.32 11 Example 12 Separator 0.45 72 155 6.20 12 Example 13 Separator 0.45 78 162 6.16 13 Example 14 Separator 0.45 83 176 6.17 14 Example 15 Separator 0.45 85 178 6.28 15 Example 16 Separator 0.45 86 181 6.40 16 Example 17 Separator 0.45 87 182 6.42 17 Example 18 Separator 0.45 86 180 6.20 18 Example 19 Separator 0.45 85 178 6.00 19 Example 20 Separator 0.45 84 177 5.94 20 Example 21 Separator 0.45 78 170 5.14 21 Example 22 Separator 0.45 70 124 4.02 22 Example 23 Separator 0.45 71 130 4.31 23 Example 24 Separator 0.45 74 134 5.09 24 Example 25 Separator 0.45 81 171 5.92 25 Example 26 Separator 0.45 84 178 6.21 26 Example 27 Separator 0.45 83 175 6.08 27 Example 28 Separator 0.45 82 170 5.85 28 Example 29 Separator 0.45 81 169 5.88 29 Example 30 Separator 0.45 80 170 5.89 30 Example 31 Separator 0.45 78 165 5.69 31 Example 32 Separator 0.45 69 102 4.01 32 Example 33 Separator 0.45 87 180 6.25 33 Example 34 Separator 0.45 85 178 6.18 34 Example 35 Separator 0.45 84 180 6.23 35 Example 36 Separator 0.45 87 182 6.39 36 Comparative Separator 0.02 58 68 4.18 example 1 37 Comparative Separator 2.00 39 42 2.19 example 2 38 Comparative Separator / 35 40 2.12 example 3 39 Comparative Separator / 60 41 2.15 example 4 40

It can be seen from Table 1 that, by controlling the ratio of the number of the first organic particles to that of second organic particles within the range defined in the present application, the rate performance, storage performance and safety performance of the battery can be significantly improved. Especially, with further optimization of the number-average particle size, the ratio of the number-average particle size of the first organic particles to that of the second organic particles or the type of the materials, the rate performance, storage performance and safety performance of the battery can be further improved. In contrast, none of comparative examples 1 to 4 satisfies the requirements of the present application, so the battery cannot have better rate performance, storage performance and safety performance at the same time.

The present inventors also did experiments by using the inorganic particles, the first organic particles and the second organic particles falling within the range of the present application, but in other amounts and with other materials, other substrates, other coating process parameters and other process conditions, and obtained the similar improvements in terms of cycling performance and safety performance of the batteries to those in examples 1-36.

Described above are merely specific embodiments of the present application, and the scope of protection of the present application is not limited thereto; any equivalent modification or replacement can be readily conceived by a person skilled in the art according to the technical range of the disclosure of the present application, and shall fall within the protection scope of the present application. Therefore, the scope of protection of the present application shall be determined by the claims. 

What is claimed is:
 1. A separator, comprising: a substrate; and a coating provided on at least one surface of the substrate; and, wherein the coating comprises inorganic particles and organic particles, wherein the organic particles comprise first organic particles and second organic particles both embedded in the inorganic particles and forming protrusions on the surface of the coating, wherein the first organic particles are secondary particles, and a number of the first organic particles in the coating is denoted as A; the second organic particles are primary particles and a number of the second organic particles in the coating is denoted as B; with the separator satisfying: 0.05≤A/B<1.
 2. The separator according to claim 1, wherein the separator satisfies: 0.2≤A/B≤0.6.
 3. The separator according to claim 1, wherein the first organic particles have a number-average particle size of ≥8 μm.
 4. The separator according to claim 1, wherein the second organic particles have a number-average particle size of ≥2 μm.
 5. The separator according to claim 1, wherein a ratio of a number-average particle size of the first organic particles to that of the second organic particles is ≥2.0.
 6. The separator according to claim 1, wherein the separator further satisfies one or more of the following (1)-(4): (1) a mass percentage of the inorganic particles in the coating is ≤75%; (2) a mass percentage of the first organic particles in the coating is ≥12%; and (3) a mass percentage of the second organic particles in the coating is ≤10%; and (4) a separator has a single-sided coating weight per unit area of ≤3.0 g/m².
 7. The separator according to claim 1, wherein the first organic particles comprise one or more of a homopolymer or copolymer of a fluorine-containing olefine monomeric unit, a homopolymer or copolymer of an olefine monomeric unit, a homopolymer or copolymer of an unsaturated nitrile monomeric unit, a homopolymer or copolymer of an alkylene oxide monomeric unit, and modified compounds of these homopolymers or copolymers.
 8. The separator according to claim 1, wherein the first organic particles comprise one or more of a vinylidene fluoride-trifluoroethylene copolymer, a vinylidene fluoride-hexafluoropropylene copolymer, a vinylidene fluoride-trifluoroethylene-hexafluoropropylene copolymer, a vinylidene fluoride-hexafluoropropylene-acrylic acid copolymer, a vinylidene fluoride-hexafluoropropylene-acrylate copolymer, and modified compounds of these copolymers.
 9. The separator according to claim 1, wherein the second organic particles comprise one or more of a homopolymer or copolymer of an acrylate monomeric unit, a homopolymer or copolymer of an acrylic monomeric unit, a homopolymer or copolymer of a styrene monomeric unit, a polyurethane compound, a rubber compound, and modified compounds of these homopolymers or copolymers.
 10. The separator according to claim 1, wherein the second organic particles comprise one or more of a butyl acrylate-styrene copolymer, a butyl methacrylate-isooctyl methacrylate copolymer, an isooctyl methacrylate-styrene copolymer, a methacrylate-methacrylic acid-styrene copolymer, a methyl acrylate-isooctyl methacrylate-styrene copolymer, a butyl acrylate-isooctyl acrylate-styrene copolymer, a butyl acrylate-isooctyl methacrylate-styrene copolymer, a butyl methacrylate-isooctyl acrylate-styrene copolymer, a butyl methacrylate-isooctyl methacrylate-styrene copolymer, a styrene-acrylonitrile copolymer, a styrene-butadiene-acrylonitrile copolymer, a methyl acrylate-styrene-acrylonitrile copolymer, an isooctyl methacrylate-styrene-acrylonitrile copolymer, a styrene-vinyl acetate copolymer, a styrene-vinyl acetate-pyrrolidone copolymer, and modified compounds of these copolymers.
 11. The separator according to claim 1, wherein the inorganic particles comprise one or more of boehmite (γ-AlOOH), aluminum oxide (A1 ₂O₃), barium sulfate (BaSO₄), magnesium oxide (MgO). magnesium hydroxide (Mg(OH)₂), silicon dioxide (SiO₂), tin dioxide (SnO₂), titanium oxide (TiO₂), calcium oxide (CaO), zinc oxide (ZnO), zirconium oxide (ZrO₂), yttrium oxide (Y₂O₃), nickel oxide (NiO), cerium oxide (CeO₂), zirconium titanate (SrTiO₃), barium titanate (BaTiO₃), and magnesium fluoride (MgF₂).
 12. The separator according to claim 1, wherein the separator satisfies one or more of the following (1)-(5): (1) the separator has an air permeability of 100 s/100 mL-300 s/100 mL; (2) the separator has a transverse tensile strength (MD) of 1500 kgf/cm²-3000 kgf/cm²; and (3) the separator has a longitudinal tensile strength (TD) of 1000 kgf/cm²-2500 kgf/cm²; (4) the separator has a transverse elongation at break of 50%-200%; and (5) the separator has a longitudinal elongation at break of 50%-200%.
 13. The separator according to claim 1, wherein the inorganic particles and the organic particles form a non-uniform pore structure in the coating.
 14. The separator according to claim 1, wherein a spacing between any two adjacent inorganic particles is denoted as L1, and a spacing between any inorganic particle and an adjacent organic particle is denoted as L2, with L1<L2. 15, A method for preparing a separator of claim 1, comprising the steps of: (1) providing a substrate; (2) providing a coating slurry, comprising a component material and a solvent, wherein the component material comprises inorganic particles and organic particles, and the organic particles comprise first organic particles and second organic particles; and (3) coating at least one side of the substrate from step (1) with the coating slurry from step (2) so as to form a coating, and drying same to obtain the separator; wherein the dried coating comprises the inorganic particles, the first organic particles and the second organic particles which first organic particles and the second organic particles are embedded in the inorganic particles and form protrusions on the surface of the dried coating; the first organic particles are secondary particles, and the number of the first organic particles in the dried coating is denoted as A; the second organic particles are primary particles and the number of the second organic particles in the dried coating is denoted as B; with the separator satisfying: 0.05<A/B<1.
 16. The method according to claim 15, wherein the method satisfies one or more of the following (1)-(8): (1) in step (2), the first organic particles are added in a mass percentage of 12% or more of the total dry weight of the component material; (2) in step (2), the second organic particles are added in a mass percentage of 10% or less of the total dry weight of the component material; (3) in step (2), the coating slurry has a solid content of 28%-45%; (4) in step (3), the coating is carried out using a coating machine, which comprises a gravure roller having a number of lines of 100 LPI-300 LPI, optionally 125 LPI-190 LPI; (5) in step (3), the coating is carried out at a speed of 30 m/min-90 m/min, optionally 50 m/min-70 m/min; (6) in step (3), the coating is carried out at a line speed ratio of 0.8-2.5; (7) in step (3), the drying is carried out at a temperature of 40° C. to 70° C.; and (8) in step (3), the drying is carried out for a period of 10 s-120 s.
 17. A secondary battery, comprising a separator according to claim 1, or a separator prepared by a method according to claim
 1. 